Received 06 May 2011Accepted 20 June 2011Published online 08 July 2011

Acknowledgments:

The authors acknowledge the support from NRF-CRP grants R-143-000-360-281 “Graphene and Related Materials and Devices” and “Tailoring Oxide Electronics by Atomic Control”, the NUS FRC grant R-143-000-440-112 and NUS YIA Grant of R143-000-452-101.